Low Melting Temperature Solder Alloy

ABSTRACT

A solder composition is provided. The solder composition consists essentially of from about 6.0 to 7.5 percent by weight of bismuth, from about 0.5 to 0.7 percent by weight of copper, and the remainder of the composition being tin.

TECHNICAL FIELD

The following relates generally to a low melting temperature solder alloy.

BACKGROUND

Solder alloys may be used to make a permanent electrical connection between two conductors. For example, a copper wire may be soldered to a lead of a capacitor. The soldering process is typically accomplished by heating the solder to above its melting point, surrounding the leads to be connected with molten solder, and allowing the solder to cool. Solders are also used to interconnect semiconductor devices including integrated circuit chips fabricated on a silicon wafer. Typically, an array of solder bumps are deposited on the top side of the wafer, the chip is flipped such that the solder bumps align with matching pads on a substrate and the system is heated to flow the solder.

Some chips, including integrated circuit chips, may be damaged by excessive heat. Because the entire assembly is heated to flow the solder in flip chip connecting methods, the melting point of the solder must be low to prevent sensitive components from being damaged.

Traditionally, lead containing solders, for example, tin-lead solders, were used, as these solders have sufficiently low melting points to minimize damage to sensitive components. However, lead and many lead alloys are toxic. Due to increasingly strict worldwide environmental regulations lead solders must be replaced with less toxic counterparts that also exhibit low melting points and sufficient conductivity in electronics applications.

Although some lead-free solders are known, these solders typically require processing temperatures that are 30 to 40 degrees Celsius higher than those historically used for production with tin-lead solders. For example, typical lead-free solders have a minimum processing temperature of about 232° C.

SUMMARY

In one aspect, a solder composition is provided. The solder composition consists essentially of from about 6.0 to 7.5 percent by weight of bismuth, from about 0.5 to 0.7 percent by weight of copper, and the remainder of the composition being tin.

In an example embodiment, the bismuth composition is approximately 6.5 percent. In another example embodiment, the bismuth composition is approximately 7 percent. In an example embodiment, the copper composition is approximately 0.7 percent.

In another aspect, the use of the solder described herein is provided for soldering metallic contacts, including copper contacts. In an example embodiment, the solder, when molten, partially dissolves the metallic contact, for example, the copper contact. In an example embodiment, the solder composition is hypoeutectic when the copper contact has been partially dissolved in the solder. In an example embodiment, the use of the composition on a Tg 140° C. laminate substrate is provided.

In another aspect a circuit board comprising the solder described herein is provided. In yet another aspect, an electronic device is provided comprising the solder described herein is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only with reference to the appended drawings wherein:

FIG. 1 is a calculated phase diagram of a tin-bismuth-copper ternary alloy;

FIG. 2 is an enlarged view of the low copper, low bismuth corner of the phase diagram of FIG. 1;

FIG. 3 is an SEM micrograph of a 92.3% tin, 7% bismuth, 0.7% copper ternary alloy;

FIG. 4 is an enlarged SEM micrograph of the alloy of FIG. 3;

FIG. 5 is an SEM micrograph of a hypereutectic 92.2% tin, 7% bismuth, 0.8% copper alloy;

FIG. 6 is an enlarged SEM micrograph of the alloy of FIG. 5;

FIG. 7 is a micrograph of the alloy of FIG. 3; and

FIG. 8 is a micrograph of the alloy of FIG. 5.

DETAILED DESCRIPTION

The present invention provides a low melting temperature solder alloy. The alloy provided is a lead-free ternary tin-bismuth-copper alloy that may be resistant to mechanical stress. The solder comprises 6.0 to 7.5 percent bismuth, 0.5 to 0.7% copper, and the remainder being tin. The solder exhibits a mechanical resistance to shock, for example, shock caused by dropping a consumer electronic device. In one example, the solder comprises 7 percent bismuth, 0.7 percent copper, and the balance of tin. In another example, the solder comprises 7 percent bismuth, 0.5 percent copper, and the balance of tin. In another example, the solder comprises 7 percent bismuth, 0.6 percent copper, and the balance of tin. In another example, the solder comprises 6.5 percent bismuth, 0.7 percent copper and the balance of tin.

Mechanical resilience of solder joints, in particular drop resistance, may be highly dependent on whether a brittle intermetallic exists within the solder joint. Intermetallic species present in solder joints typically form when the solder alloy is cooled from its molten state. For example, a ternary tin-bismuth-copper alloy may form a brittle Cu₆Sn₅ intermetallic when cooling if the copper content is above the eutectic composition. It is therefore important to ensure that the copper content is sufficiently low to prevent brittle intermetallic species from forming in a solder joint.

Due to the relatively low melting point of the solder alloy composition with respect to other lead-free solder compositions, the solder alloy provided is more compatible with heat sensitive parts. The solder alloy is characterized by a minimum processing temperature of approximately 222° C. The minimum processing temperature is sufficient for soldering components with leads, also known as “leaded components”, as well as ball grid array connections. Leaded components typically comprise tin on the immediate soldering surface, with copper or other alloys also contributing to the solder joint. The low melting temperature of the solder composition may reduce overheating during solder processing steps. Additionally, the lower soldering temperature relative to conventional lead free solders may reduce circuit board mechanical failure modes such as delamination, warpage, and open solder joints, also known as head-in-pillow.

The low melting point of the solder may enable the use of less heat-resistant circuit board materials and other electronic components, which are typically less brittle at room temperature. For example, the solder composition provided herein may be used with laminate boards having a glass transition temperature of 140° C. and known as Tg140° C. laminates.

Tg 140° C. laminates are well established within the electronics industry and the performance of the boards with respect to soldering temperature is well characterized. Tg 140° C. laminates are generally reliable in electronics products, as these laminates are less brittle and less susceptible to the pad cratering failure mode, which is a mechanically-induced fracture in the resin of the laminate between the outermost layer of fibreglass and copper foil. In contrast, materials typically used in standard lead-free processes have a higher glass transition temperature of 170° C. and are known as Tg 170° C. laminates. The low melting point of the solder reduces production costs, as Tg 140° C. circuit boards are less expensive than Tg 170° C. circuit boards.

Referring now to FIG. 1, a two dimensional view of the ternary phase diagram of a calculated tin-bismuth-copper ternary system is shown. It will be appreciated that the full phase diagram is three dimensional and the diagram of FIG. 1 represents only a portion of the full diagram. As can be seen at numeral 100 on the diagram, at bismuth concentrations below about 8 percent and at copper concentrations below about 0.8 percent, the melting point of the ternary alloy is approximately 220° C. It can also be appreciated from the diagram that slight variations in the mass percent of copper in the solder alloy may have an appreciable effect on the structure and melting temperature of the alloy.

Referring now to FIG. 2, an enlarged view of the highlighted portion 100 of the diagram of FIG. 1 is provided. Shaded region 202 outlines the area of the diagram corresponding to the approximate solder composition of the present invention. As can be seen from FIG. 2, the area of the phase diagram corresponding to the approximate solder composition described herein is near-eutectic. A near-eutectic composition is a composition that is near the eutectic. In this case, the composition is slightly below the eutectic. If the copper content of the solder composition of FIG. 2 were increased, the solder composition may rise above the eutectic.

As mentioned above, if the solder composition is hypereutectic, the solder will form brittle intermetallic species. It is therefore important to ensure that the solder composition is eutectic or slightly hypoeutectic when cooling. Importantly, if the solder alloy is cooled very quickly, it is possible that brittle intermetallic species could form even in a hypoeutectic composition. It is for this reason that in most applications where the cooling rate of the solder cannot be practicably controlled, that a solder having a slightly hypoeutectic composition may be used.

When the solder is heated to its molten state and brought in contact with a copper surface, the solder may dissolve a portion of the copper surface. The dissolved copper enters into the solder composition, thereby increasing the weight percent composition of copper in the solder alloy. Therefore, for applications where the molten solder will come in contact with copper, it is important that the solder composition is hypoeutectic to account for the solubilised copper during the soldering process. The percent composition of copper in the alloy may be varied depending on the intended use of the solder. For example, when soldering two copper contacts, it may be desirable to use a solder composition with a lower copper content, for example, 0.5% Cu. Conversely, when soldering other metal contacts, it may be desirable to have a solder composition with a comparatively high copper content, for example, 0.7% Cu.

Referring back to FIG. 2, the shaded region 202 is slightly hypoeutectic, enabling the solder composition to solubilise some copper without rendering the composition hypereutectic. Moreover, the hypoeutectic composition of the solder enables the solder to cool rapidly without forming brittle intermetallic species.

A solder composition comprising 7.0 percent bismuth, 0.7 percent copper and 92.3 percent tin typically forms a hypoeutectic even after being molten in contact with copper and cooling at a rate consistent with typical processing conditions. Turning now to FIG. 3, it can be seen from the scanning electron microscope (SEM) micrograph of the polished surface of this solder alloy 300 that the alloy is substantially uniform and no brittle intermetallic Cu₆Sn₅ has been formed. The uniform morphology of the alloy shown in FIG. 3 may be favourable in terms of mechanical properties including drop shock resistance. An enlarged SEM image of the alloy of FIG. 3 at approximately 1000 × magnification is provided in FIG. 4. As can be seen in FIG. 4, the surface 300 of the alloy is substantially uniform and without intermetallic deposits.

However, if the copper concentration of the alloy is increased from 0.7 percent to 0.8 percent, intermetallics may form as the solder composition cools. Referring to FIG. 5, an SEM micrograph of a solder composition comprising 7.0 percent bismuth, 0.8 percent copper, and 92.2 percent tin is shown. Although the bulk of the alloy 300 appears similar to that of FIG. 3 and FIG. 4, the brittle intermetallic species 502 comprising Cu₆Sn₅ compromise the mechanical integrity of the solder.

Turning now to FIG. 6, an enlarged SEM image of a brittle intermetallic species 602 is shown on the surface of a solder having a composition identical to that of FIG. 5. As can be seen, the brittle intermetallic species 602 may be formed with straight edges, due to its crystallographic structure. Furthermore, several cracks have formed in the intermetallic species 602, as is outlined by numeral 602. The cracks 602 may have been formed during preparation of the micrograph sample, however, the cracks 602 are a representative visual indication of the brittleness of intermetallic species.

A micrograph of the solder composition of FIG. 3 is provided in FIG. 7. The surface of the micrograph 300 is uniform and substantially free from contaminants, blemishes, and intermetallic species. In contrast, the micrograph of FIG. 8 shows two large intermetallic crystals 802 extending deep into the sample. As is clear from the micrograph, the intermetallic crystals 802 are defined by approximately straight edges. The surface of the solder alloy on the interface with the intermetallic crystals 802 would be a probable location of crack initiation, particularly if a solder comprising large intermetallic crystals 802 is dropped on a hard surface to produce a substantial impact. Due to the likelihood of high stress intensity ratios in the interface between the intermetallic crystal 802 and the solder alloy being greater than the bulk alloy, the area around intermetallic crystals may also be more susceptible to crack propagation from repeated stresses or impacts than the bulk material.

It will be appreciated that certain features of any of the described embodiments could be applied to other embodiments described herein. It will also be appreciated that although the solder composition was described with reference to soldering copper contacts, the solder composition may be used to solder other materials, and other conductive metals in particular.

Although the above has been described with reference to certain specific example embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the scope of the claims appended hereto. 

We claim:
 1. A solder composition consisting essentially of: from about 6.0 to 7.5 percent by weight of bismuth; from about 0.5 to 0.7 percent by weight of copper; and the remainder of the composition being tin.
 2. The solder of claim 1 wherein the bismuth composition is approximately 6.5 percent.
 3. The solder composition of claim 1 wherein the bismuth composition is approximately 7 percent.
 4. The solder composition of any one of claims 1 to 3 wherein the copper composition is approximately 0.7 percent.
 5. The solder of claim 1 wherein the solder is hypoeutectic.
 6. A use of the solder composition of claim 1 for soldering metallic contacts.
 7. The use of claim 6 wherein the solder is used for soldering copper contacts.
 8. The use of claim 6 whereby the solder, when molten, partially dissolves the metallic contact.
 9. The use of claim 7 whereby the solder, when molten, partially dissolves the copper contact.
 10. The use of the solder of claim 9 whereby when the copper contact is partially dissolved, the solder is hypoeutectic.
 11. The use of the composition of claim 1 with a Tg 140° C. laminate circuit board.
 12. An electronic device comprising the solder of claim
 1. 